Symbol synchronization in a digital communication receiver refers to identifying the instants in time at which samples of an input communication signal are best obtained to recover data conveyed by the input communication signal. Only one sample is needed per symbol interval to accurately recover communicated data. A symbol interval, also called a unit interval or simply a symbol, is a discrete duration within which a received signal conveys a unit of data. The unit of data may include one or more bits. The process of symbol synchronization determines the best instant within each symbol interval at which to obtain a sample that will be relied upon in the recovery of the unit of data.
A few prior digital receivers have achieved symbol synchronization using only one complex sample per symbol interval. However, such digital receivers have been extremely sensitive to carrier synchronization and frequency offsets, such as those caused by Doppler. The majority of prior digital receivers achieves symbol synchronization using two or more complex samples per symbol interval. The use of two or more complex samples per symbol interval is highly undesirable because it requires the use of high speed components and/or the use of excessively complex parallel circuits.
U.S. Pat. No. 5,671,257, assigned to the assignee of the present invention, teaches a magnitude-based symbol synchronizer that achieves symbol synchronization using as few as one complex sample per symbol interval. This magnitude-based symbol synchronizer works well for many applications without modification. However, when the received communication signal has experienced significant distortion, e.g. excessive multipath, the magnitude-based symbol synchronizer of U.S. Pat. No. 5,671,257 may upon occasion unreliably track symbol timing of the received signal, and may even fail to lock to the symbol timing in extreme situations.
Accordingly, a need exists for a symbol synchronizer that performs robustly, even in the presence of significant received signal distortion.